I've been in this malarkey for the last 40 years or so. Longer in fact, so welcome to my teetotal world. I'm also not too fond of diet soda drinks, the acidic sweeteners zing straight on my tongue on contact. So yes, water, tea, coffee and above all sparkling water. Over time you can tell the difference between difference waters and between soda water and sparkling water. There is a Fever Tree tonic water which is "naturally light", it uses fructrose rather than sweeteners, so no zing, and that is just under 3% sugars, compared to 7% for the main tonic or 12% for something like ginger ale. One thing I sometimes do is mix a small amount of apple juice into sparkling water. The sugar tax unfortunately means that things like bitter lemon and ginger beer now have some sweeteners in them but it's much less than in true diet drinks so you can perhaps dash a small 100 ml tin of bitter lemon over 4 or 5 glasses of sparkling water. You will need to talk to the crew about this since in CW the service standard is not to hand over the tin unless you ask for it.

I can only remember about 2 flights where there was no sparkling water, it rarely runs out, and in both cases I think it was due to being placed into the wrong canisters or crew lack of familiarity. I tend to buy some sparkling water in the shops in T5 or T3 if I am not connecting on to a further flight: it is intended for the hotel on arrival (many Asian locations don't really do sparkling water, even USA isn't that easy in this regard) but it's a useful emergency stock just in case.

The other good news is that the Union coffee is now much more palatable and is now available in lounges, Club Europe and Club World.